A system of air in which warm, rising air cools as it expands into higher and colder altitudes.

• A. Low-pressure system
• B. High-pressure system
• C. Air mass
• D. Wind

Answer: (A)

Warm, rising air expands as it moves into higher, colder altitudes because the external pressure drops with height. When the air expands, it does work on its surroundings and loses internal energy, so its temperature falls—this is adiabatic cooling. This vertical ascent and cooling are typical of a low-pressure system, where surface convergence forces air upward. In high-pressure systems, air tends to sink and compress, warming rather than cooling, which doesn’t match the description. An air mass is simply a large body of air with relatively uniform properties, not defined by vertical motion, and wind is horizontal movement driven by pressure differences, not the rising air process described here.